import java.util.Stack;

public class 用两个栈实现队列 {

    Stack<Integer> stack1 = new Stack<Integer>();
    Stack<Integer> stack2 = new Stack<Integer>();

    public void push(int node){
             stack1.push(node);
    }
    public int pop(){
        if (stack1.empty()&&stack2.empty()){
            throw new RuntimeException("Queue is empty!");
        }
        if (stack2.empty()){
            while (!stack1.empty()){
                  stack2.push(stack1.pop());
            }
        }
        return stack2.pop();
    }


}
